Chapter 5: Branching and merging

CVS allows you to isolate changes onto a separate
line of development, known as a branch. When you
change files on a branch, those changes do not appear
on the main trunk or other branches.

Later you can move changes from one branch to another
branch (or the main trunk) by merging. Merging
involves first running cvs update -j, to merge
the changes into the working directory.
You can then commit that revision, and thus effectively
copy the changes onto another branch.